1. Field of the Invention
The invention relates to an image sensing device comprising a solid-state image sensor with at least one radiation sensitive element to convert incident radiation into an electric charge, a read-out circuit to form a pixel signal from the electric charge, which pixel signal includes at least a hold level and an image level while the image sensing device comprises a signal processing circuit to determine a difference signal representative of the difference between the hold level and the image level and to derive an electronic image signal from the difference signal. The invention also relates to an x-ray examination apparatus which comprises an image sensing device with a solid-state image sensor.
2. Description of the Related Art
An image sensor of said kind is known from the U.S. Pat. No. 4,454,435.
The signal processing circuit of the known image sensor is arranged so as to form an electronic image signal from the pixel signal by sampling a brief sample portion of the hold level and another brief sample-portion of the image level. The electronic image signal representative of the pixel-value of the pixel in question is derived from a difference signal representing the difference between said sample portions. The electronic image signals of respective pixels are further amplified and combined to form an electronic video signal which is applied to an output circuit, which has the form of an output pad. Consecutive pixel signals which relate to different pixels of an image are separated by a reset-pulse at the start of each pixel signal. The reset-pulse is caused by cross-talk via parasitic capacitances when the gate of a reset transistor is switched to clear the read-out circuit before an electric charge is read-out, a comparison voltage being applied to its source contact, and the drain contact being connected to the output gate of the read-out circuit which supplies the electric charges. Both sample-portions contain the same noise contribution which is induced by the switching on of the reset transistor. Therefore, although the reset transistor produces thermal noise, the thermal noise component of the electronic image signal is partly reduced. Since only brief sample-portions are employed for the formation of the difference of these sample portions, the resulting electronic image signal will still contain a substantial amount of noise. Moreover, variations of residual charges in the read-out circuit may also add noise to the difference signal. As a consequence, the known image sensor is not suitable for recording incident radiation of low intensity. The electronic image signal formed by the known image sensor when an image of low intensity is recorded will have a signal-to-noise ratio which is not sufficient to discern relevant details in the recorded image.
In x-ray examination procedures especially, insufficiency of the signal-to-noise ratio of the electronic image signal is a serious drawback. Since x-rays are to some degree harmful to human tissue it is desirable to reduce the x-ray intensity as much as possible; the luminous intensity of a visible light image derived from an x-ray image is lowered correspondingly. When such a light image is recorded by the known image sensor, the resulting electronic image signal would have a signal-to-noise ratio which is insufficient for forming on a monitor or on a hard copy an image which has an adequate image quality for a physician to make a diagnosis.